t, f = hyper_as_trig(e) should preserve f(t) = e invariant

This was not the case for constant expressions, see #7761.  Closes #7761.

@smichr
On Sun, Jul 20, 2014 at 08:50:39AM -0700, Sudhanshu Mishra wrote:
>    In sympy/simplify/tests/test_simplify.py:
> 
>  > @@ -293,6 +293,10 @@ def test_trigsimp_issue_4032():
>  >          2**(n/2)*cos(pi*n/4)/2 + 2**n/4
>  >
>  >
>  > +def test_trigsimp_issue_7761():
>  > +    assert trigsimp(cosh(pi/4)) != sqrt(2)/2
> 
>    Instead of testing x != y we can do x == z.

Fine.  My question was: what will be x and z?